1. Field of the Invention
This invention relates to a method of producing a discharge display device and more particularly to a method of forming a LaB.sub.6 cathode for the discharge display device.
2. Description of the Prior Art
Recently, development of discharge display devices, especially direct current type XY matrix discharge display panels termed plasma display panel or PDP has been promoted. In such a discharge display panel, Nickel (Ni) is usually used as an anode and a cathode. However, Ni has insufficient resistance against discharge sputtering, and therefore a Ni cathode deteriorates in several seconds of operation. To cope with this, mercury (Hg) has been sealed in the discharge display panel and deposited on a surface of the electrode to suppress sputtering.
On the other hand, a direct current type discharge display panel developed by the present inventors employs a unique driving system, that is, a trigger discharging system, and when it is applied to an XY matrix panel with a large capacity, it is necessary to provide discharge characteristics, (i.e., the characteristics of trigger discharge and main discharge) of each display cell uniform to a certain degree. However, in a discharge display panel having mercury (Hg) sealed therein, a non-uniform distribution of the mercury commonly occurs due to change on tanding, and it is difficult to retain uniform discharge characteristics for a long time. For this reason, it is important to provide a discharge display panel in which no mercury is sealed. Further, for example, where a discharge display panel is to be used in a closed room such as a cockpit, mercury should not be used in consideration of danger.
Further, in the XY matrix type discharge display panel, it is generally important to attain reduction in power consumption, long life, high discharge efficiency and reduced driving voltage, etc. Meanwhile, lathanum boride (LaB.sub.6) has been noticed as a cathode material. LaB.sub.6 is low in its discharge holding voltage, and is stable in physical and chemical properties, thus meeting the above-mentioned requirements.
However, a LaB.sub.6 cathode has not yet reached practical use for the reason that production employing a thin-film evaporation method or a plasma spraying method is complicated and results in increase in cost. Particularly, it is difficult to form a relatively uniform electrode with a large capacity and a large screen. Another reason is that the electrode cannot be formed in connection with the other panel structure by a thick-film printing method with a low cost.
In a case where the LaB.sub.6 cathode is intended to be formed by the thick-film printing method, it is generally burnt in an atmosphere of nitrogen of N.sub.2 at 800.degree. C.-900.degree. C. after printing and application. However, as a substrate of the discharge display panel is glass, temperature is permitted to be raised up to about 600.degree. C., and as a structure such as the other electrodes and a barrier is oxide, a burning step is usually carried out in the air. For these reasons, it is difficult to form the LaB.sub.6 cathode. In addition, LaB.sub.6 has a high melting point of about 2300.degree. C., and therefore it cannot be sintered at a temperature of about 600.degree. C., with a result that resistance after formation of the cathode is disadvantageously increased to 10.sup.9, and more. In the case that the thick-film printing method is adopted, a binder substance such as frit glass is generally mixed with LaB.sub.6 powder so as to obtain a bonding strength between particles of the LaB.sub.6 powder. However, it is considered not possible to use a mixture of such glass binder with LaB.sub.6 powder, due to the resulting high resistance after formation of the LaB.sub.6 cathode.